1. Field of the Invention
Polycrystalline articles which contain silicon nitride and which can be produced by sintering with or without application of pressure from starting powders containing finely dispersed silicon nitride, and appropriate aluminum nitride and/or alumina and other oxides, are known. Owing to their properties of such high strength at room temperature, high impact resistance, low thermal expansion and good wear properties, they can be used in many fields of application.
2. Description of the Related Art
Because of the limited scope of shaped articles provided by conventional hot-pressing processes, pressureless sintering processes are preferred for the production of components of complex shape wherein the starting powders must be compacted to form green compacts of the desired final shape before they can be sintered by heating to the required temperatures.
Among the known molding processes for the production of green compacts in the form of, for example, thin plates, rods, thin-walled crucibles or tubes, extrusion processes, injection-molding processes and especially slip-casting processes have proved useful. It is important that green compacts of adequate stability are produced so that they can be mechanically worked before sintering, since working after sintering is, because of the hardness of the silicon nitride-based sintered material, very expensive and possible only with diamond tools.
A reference to slip-casting of ceramic materials based on silicon nitride is found in GB-A-1,052,590. In this case, organic solvents are used exclusively as a liquid dispersion media in which the polymeric cyclopentadiene, used as a defluocculant, is readily soluble. In the examples, however, no data on the sintered densities obtained were given for the use of silicon nitride powder with xylene as the dispersion medium and subsequent sintering. Moreover, it was pointed out that, when water was used as the dispersion medium, it was virtually impossible to obtain castings of satisfactory properties because most of the ceramic materials contain impurities which react with water to form by-products which adversely affect the physical cohesion of the resulting green compacts.
The injection-molding of ceramic powders which are based on silicon nitride and contain up to 30% by weight of yttrium compounds or compounds of the lanthanides, in addition to trace quantities present in the raw material, of Fe, Ca, Mg or compounds thereof, which are processed together with an organic binder to form green compacts, is disclosed in U.S. Pat. No. 4,296,065.
A further reference to the possible slip-casting of ceramic powders on a sialon base, such as Si.sub.3 N.sub.4, AlN and/or Al.sub.2 O.sub.3, together with sinter aids such as Y.sub.2 O.sub.3 or oxides of the lanthanides, a dispersion of a solids content of 50-70% by weight in water being recommended as advantageous, can be derived from U.S. Pat. No. 4,184,884.
In the meantime, the view has generally gained acceptance that success or failure both in the slip-casting process and in the injection-molding process depend largely on the nature of the organic binder used.
Good cohesion of the individual powder particles can be achieved by means of temporary organic binders. The temporary binders must be removed as a result of which defects or cavities are formed in the sintered compact before or at the latest during the actual sintering process. Cracked locations or fracture locations are pre-programmed. With water alone as the binder or dispersant, there is a risk of ceramic powders based on silicon nitride and/or aluminum nitride reacting with the water with decomposition (cf. DE-C-3,525,752).
The properties of water-based slip-casting mixtures of silicon nitride powder and oxidic sinter aids and inorganic additives have been investigated. Green densities of about 50% of the theoretical density were achieved (cf. E. M. Rabinovich et al., J. Mater. Sci. 1982, 17(2), pages 323-328; abstracted in C.A. Volume 96, 1982, No. 204, 230 s, and M. Persson et al., J. Mater. Sci. Monogr. 1983, 16 (Ceram. Powders), pages 735-742; abstracted in C.A. Volume 99, 1983, No. 75,586 y).
In investigations into the possible influence of sinter aids on the slips, it was found that after the addition of magnesium compounds (MgO or Mg(HCO.sub.3).sub.2), the viscosity increased considerably. In order to obtain a castable suspension, dilution was necessary which then led to green compacts of only 35 to 40% of theoretical density (cf. W. J. A. M. Hartmann et al., J. Phys. Colloq. 1986, pages C1-79-C1-83; abstracted in C. A. Volume 104, 1986, No. 191,475 g).
It is also known to use, instead of temporary organic binders, those binders which can be hydrolyzed or decomposed with the formation of a temporary bond which is then not removed in the firing process but can be converted into a refractory oxide bond. As examples of such binders, alkyl silicates were mentioned which form an SiO.sub.2 bond in the firing process. Organic aluminum compounds or magnesium oxychloride or magnesium oxysulfate, which can form an aluminum oxide bond or magnesium oxide bond, were also mentioned (cf. GB-A-1,054,631). As can be seen from the examples, however, only highly porous end products having a porosity of 44%, or 32%, corresponding to approximately 56-68% of theoretical density, were obtained when a mixture of Si.sub.3 N.sub.4 powder and ethyl orthosilicate in aqueous ethanol solution was cast and the green compacts then fired at temperatures of up to 1350.degree. C. The formation of a refractory oxide bond alone, expected under the indicated conditions, was insufficient to obtain sintered silicon nitride products of high final density.
The state of the art is that, for the production of silicon nitride-based ceramic masses which are to be processed for molding by the slip-casting process, injection-molding process or extrusion process, organic binders and/or organic dispersants are preferred, since relatively good cohesion of the ceramic powder particles before the sintering process is ensured, but at the price of poorer properties of the finished sintered product.
By contrast, using water and/or inorganic additives, only relatively low density green compacts are obtained, which are not sufficiently stable for mechanical working before the actual sintering process.
The processing of water-based ceramic mixtures is preferred to processing based on organic additives, for economic and environmental reasons, and to avoid interfering influences on the properties of the finished sintered products. It is desirable to provide a process for producing green compacts by molding according to the slip-casting process, injection-molding process or extrusion process from sinterable ceramic mixtures based on dispersants and binders containing starting powder mixtures, which contain silicon nitride, if appropriate aluminum nitride and/or alumina and other oxides as sinter aids, and water. Such a process guarantees not only an adequate stability for mechanical working and a relatively high density of the green compacts, but also an interference-free sintering step, producing homogeneous, polycrystalline sintered products having high final densities.